(1) Field of the Invention
The present invention relates to improved fume hoods or ventilated workstations, and in particular to fume hoods that include a highly sensitive air velocity regulator to measure and maintain the minimal face velocity necessary to prevent exhausting of the hood gases through the hood opening.
(2) Description of the Prior Art
Fume hoods or vented workstations are used in laboratories and other environments to manipulate materials that might generate noxious or dangerous gases or fumes without releasing the materials or components or fumes therefrom, into the work environment. Generally these workstations are comprised of an enclosure or chamber in which materials are handled, and means for drawing air through a front opening in the enclosure that is also used by the operator as means of access into the enclosure. The enclosure also includes an exhaust opening, frequently communicating with a filter to remove contaminants from air exhausted from the chamber.
The fume hood is normally comprised of side and top walls, which may be of transparent material, such as Plexiglas.TM. or other clear plastic, a rear wall with an exhaust opening, and a planar bottom wall or floor. The front edges of the top, side and bottom walls may form an operator access opening. A slidable door may be positioned to cover, or vary the size of, the access opening. Generally, the hood is configured, and air vanes are often added, so that air enters the access door and is exhausted through the exhaust opening, with generally laminar air flow being maintained within the chamber, preventing turbulence that could disturb the materials being manipulated.
Escape of contaminated air from the hood chamber through the access opening into the work environment is prevented by maintaining a pressure differential between the chamber, or hood interior, and the work environment, or hood exterior, so that air continually flows from the hood exterior through the access opening into the hood interior. A sufficient air velocity at the access opening, known as the "face velocity," must be maintained to prevent contaminated air from escaping. From an economic standpoint, however, it is important to not employ an air flow velocity greater than is necessary.
Contaminated air is exhausted from a fume hood through an exhaust conduit that includes a vacuum source to draw the air through the exhaust conduit. Generally, this vacuum source is comprised of an exhaust fan positioned within the conduit, and an electric motor to turn the fan. The gas may be exhausted to the exterior environment. In many instances, however, the air will be conveyed through a filter, such as a HEPA filter, to remove contaminants from the air.
The cost of operating a fume hood is attributable to the cost of electricity necessary to maintain the external environments, and to operate the exhaust motor. The initial cost of the hood will be largely dependent upon the size of the motor and other components of the exhaust system that is required. Since exhaust fans may run more or less constantly, a small difference in the size or speed of the motor can make a significant difference in the hood initial cost and its operating cost. Therefore, it is important to minimize the volume of air that flows through the hood chamber during operation.
Air flow is normally controlled by sensing the air velocity at the access opening or inlet into the hood, i.e., the face velocity, to maintain a velocity sufficient to prevent leakage of contaminated air. The measured velocity is then compared against a desired velocity, and a signal is transmitted to a control mechanism that adjusts the rate of air flow through the exhaust conduit. The flow rate can be adjusted by varying the motor and fan speed. However, since the velocity needs to be quickly adjusted, the control mechanism normally changes the position of a valve or damper within the exhaust conduit.
Various prior art hoods and control mechanisms incorporate these components.
For example U.S. Pat. No. 5,415,583 to Brandt, Jr. describes a fume hood and air flow control system comprised of a hood having an air access opening, a sensing means at the opening to determine face velocity, an exhaust means that includes a conduit and a valve positioned in the conduit, a pressure comparator, and an actuator. The sensing means is a pitot that reads total and static pressures. These readings are transmitted to the comparator, which in turn sends a signal to the actuator. The actuator then adjusts the valve position to change the air flow through the hood chamber.
The Brandt pitot is basically a pair of parallel tubes joined by a pair of parallel plates, with each plate having front and rear edges that are attached to the tubes. The front tube, e.g., the tube to the exterior of the chamber, includes a plurality of holes at the front of the tube extending through the tube wall into the tube interior. The rear tube, e.g., the tube to the interior of the chamber, also includes a plurality of holes that extend vertically downward through a plate and the tube wall into the tube interior. The hood includes a pair of spaced parallel, curved airfoils stacked above the lower edge of the access opening with the pitot being positioned between, and spaced from, the airfoils.
While the Brandt, Jr. patent and other prior art patents describe hood air velocity sensors and flow control mechanisms in fume hoods including these sensors, there is a continuing need for further improvements in air flow sensors and mechanisms that would provide greater sensitivity to fluctuations in air flow. Such sensors, by having the capability to measure air flow changes at lower air flows, would lessen the volume of air that is required to maintain a necessary face velocity, thereby reducing manufacturing and operational costs.